A possible third way, given the "buttons only" constraint, would be an up/down button pair (with auto-repeat) per digit.
Up/down + left/right for selecting the digit.
Dangerous. When looking at my scope, if I hit the 10s digit +, I can go from +5v to +15v in 1 step, or the same with current if my cursor is on the wrong digit, or, I hit the wrong 'UP' button.
To move the large values, a keypad with numbers + enter, and cancel is great.
To increase and decrease voltage and current in steps, the addition of up/down arrows is also needed, but, keep those steps at a safe manageable size. Acceleration here is ok if the up/down button is held for more than 3-5 increment steps.
I prefer both the keypad + at least 1 dedicated set of up/down arrows + 1 enable/disable output button.